The present invention relates to a method for finishing a surface of a workpiece, especially inner walls of bores of an internal combustion engine, wherein a finished workpiece surface is produced that is highly resistant with respect to friction and temperature and wherein the workpiece surface during operation of the workpiece is provided with a lubricant.
For generating highly resistant surfaces, for example, of inner walls of bores which serve as the cylinder bore of piston engines, a honing technique with subsequent plateau-honing is known. In this method, a bore, which, for example has been premachined by fine-boring, is first honed to a desired dimension that is a little less (within a .mu.m range) than the required final dimension. The honing step is provided to generate a bore with high shape exactness and dimension precision and a minimal roughness depth. This operation is carried out in one or more honing steps, for example, by pre- and intermediate honing steps. During the subsequent operation of the workpiece which, for example, is comprised of cast iron, the crossed honing traces generated by the honing stones form a reservoir for the required lubricant. During the subsequent plateau-honing step the final dimension of the workpiece is produced with a very fine cutting tool whereby only the tips of the already honed surface is removed. This method step increases the supporting surface portion of the bore for supporting the piston moved therein during operation of the workpiece.
It is also known to carry out the last method step by subjecting the workpiece to a stream of a medium, and it is known to use laser beams for this purpose (German Offenlegungsschrift 39 32 328). With a laser beam the tips of the honed surfaces are removed primarily by vaporization, and furthermore loosely attached particles and rough spots can be removed. The honing traces are maintained during this treatment.
Furthermore, a final treatment of finished honed bores are known in which with brushes and/or high pressure jets the so-called "sheet-metal shell", i.e., a metallic layer generated by bent tips which covers the graphite lamellas of the material of the workpiece, or rough spots are removed and the graphite lamellas of the workpiece material are substantially exposed (U.S. Pat. No. 4,980,996). With this treatment the honing traces which are important for the subsequent lubricant distribution remain also unchanged.
Because the cutting layer of the honing stones which are being used for honing are provided with irregularities depending on the material used for the honing stones, the formation of the honing traces with respect to width, depth, and distance to one another is random. Furthermore, by using these honing tools, which commonly have six to eight honing stones distributed over their circumference, deviations of the ideal circular form of the cross-sections over the length of the bore have been observed so that the bore may be somewhat constricted at some locations relative to the exact final dimension. In a subsequent method step more material is being removed at these locations than in other areas so that the supporting surface portion of the surface of the workpiece varies over the length of the bore whereby additionally fluctuations in the depth of the honing traces occur.
It is therefore an object of the present invention to carry out the finishing of the surface of a workpiece such that a lubricant reservoir with a uniform distribution of the lubricant can be attained. Furthermore, over the length of the bore a high proportion and a uniform distribution of supporting surface area is desired.